Even though Acts 1:17 is a short verse we'll cover it in two posts, looking at the verb forms κατηριθμημένος and ἔλαχεν in more detail.
ὅτι κατηριθμημένος ἦν ἐν ἡμῖν καὶ ἔλαχεν τὸν κλῆρον τῆς διακονίας ταύτης
for numbered he was among us, and he obtained a share of this ministry
From the previous verses we know that it is Peter speaking, and that the subject here is Judas Iscariot.
Κατηριθμημένος is a hapax legomenon. It is the masculine singular nominative, perfect middle/passive participle, of the verb καταριθμέω - 'I number among'.